WARNING! This update does not generate caves, or The End, and lighting is broken!

Here's the development snapshot of week 7, 2012! List of changes:

Added new world file format called “Anvil” (256 max height and 4096 block IDs*)
Multi-player light calculations do no longer cause affected blocks to be transmitted to the clients, instead the clients will recalculate the light on their own
Villagers will repopulate villages based on how many houses there are available
Some nights in villages will be worse than others…
Added a redstone-controlled light source
Decreased chance for the rare mob drops
All animals use the new AI system now
Corrected crash bug corrupting levels when spawning mobs above the 255th block
Sheep animation is correct in SMP

* The game can't use the full range of IDs yet because there are still some assumptions that IDs above 256 are items.

NOTE: This snapshot will convert the maps you load to the new file format. If you want to revert to the old format, you need to replace the “level.dat” file with the file called “level.dat_mcr”. The new format will write world regions to files called “*.mca”, so your original regions will remain as “*.mcr”.

On that note, Jon and I have been working on a new map format for Minecraft that we've dubbed “Anvil” (it's just a name). The purpose of the new format is to prepare for modding, especially to allow for more block types, but also to improve other areas of the game (such as map height). The changes are:

-Maximum build height has been increased to 256 (was 128)
-Empty sections of the world are not loaded into memory
-Block ids have been increased to 4096 (was 256) by adding a 4 bit data layer (similar to how meta data is stored)
-Block ordering have been changed from x/z/y to y/z/x in order to improve compression
-Packets for sending chunks have been updated (a full 128-high chunk is smaller than the old format, and a chunk with lots of empty space is much smaller)
-Biomes are saved per x/z column, which means they can be altered by tools

The next snapshot, and Minecraft 1.2, will automatically convert worlds to the new format. The files of the current format (“McRegion”) will remain in your save folder, if you need to restore them for some reason.

I've prepared a converter (with source code) for tool makers that wants to be prepared. You do not need to use this to convert your maps! It's only for reference!

-The 16x128x16 “Blocks”, “Data”, “SkyLight” and “BlockLight” tags are removed
-A “Sections” tag has been added with up to 16 compound tags
-Each section has 16x16x16 “Blocks”, “Data”, “SkyLight” and “BlockLight” tags
-Each section has a “Y” integer tag saying which section it is (note that some Y positions can be skipped). 0 is at the bottom of the world and 15 is the topmost section
-Each section also has a “AddBlocks” tag, which is a DataLayer byte array just like “Data”. The “AddBlocks” tag is not included in the converter since the old format never had block ids above 255. This extra tag is created whenever a block requires it, so the getTile() method needs to check if the array exists and then combine it with the default block data. In other words, blockId = (addBlock << 8) + baseId.
-Each chunk has a 16×16 byte array with biome ids called “Biomes”. If this array is missing it will be filled when the game starts. The converter doesn't include any biome sources, though (it had too many dependencies to be included).
-Note that the old format is XZY and the new format is YZX (see the difference between DataLayer and OldDataLayer).
-The new format uses the extension “.mca” instead of “.mcr” (the old format is kept for safety, with the level.dat backed up as “level.dat_mcr”)